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Posts posted by vicious5id

  1. indulis, R1 and R5 are present to take some voltage away from the transistor so it dissipates as little power as possible, with the setup shown it only churns out ~400 mW.

    The application is basically that I have a ~20 Ohm load resistor which I expect to change in value in accordance with its temperature, but the current that I apply through it should not change, even if the resistance were to change.

    However I have tried it out a few times and the load does not look like its the problem, I think its more to do with the circuit, I have a funny feelind that the feedback might be messed up.

  2. Greets everyone,

    I am not sure if this is the right place to post this in the forum, but I have a problem with a circuit that I designed at the implementation stage.

    ^ That's the circuit that I have designed. The purpose of it is to behave as a current source that supplies a range of current from 0 to 120 mA, to a 20 Ohm load (R11 in schematic). I have simulated the circuit within software known as SIMetrix and it works as I want it to under high temperatures, long periods of operation and if the 20 Ohm load alters in value. However once I built the circuit and tried it out I was surprised.

    When I begin to rotate the pot, and stop it at a point where I need it say 52.2 mA (?!), the current will immediately begin to drop by 0.1 mA. Over time it will keep on dropping (over time = ~10 mins) but the 0.1 mA reduction gets very slow. I thought that it's becuase my resistors or something else was getting hot, however if I cooled the transistor more current began to flow!

    I really don't want the current to drift by 0.1 mA at all! I want the current to stay exactly where I set it regardless of how much temperature changes. Also the current I set should stay constant for +/- 10 Ohms in the load. So if I were to set the current I want output to 80 mA it should stay constant over a 10, 20 and 30 Ohm load.

    Well anyway I think this post is getting long enough and I don't want to start mixing my objectives and confusing everybody!

    I really appreciate any help you guys have to offer.

    v5 0ut.

  3. Hey wassup people! Man tonnes of work has gone into this circuit, I'm very impressed with all these radical ideas about thermometers and ammeter adaptations! Who'd have thought an IC first manufactured in 1977 (7106, 7107 is the sister IC) would still be so very useful today!

    Anyway I just wanted to clear up a few things:

    a) http://www.electronics-lab.com/projects/test/007/index.html
    b) http://www.electronics-lab.com/projects/test/014/index.html

    ^ Are those two circuits exactly the same, if not do they accomplish the exact same thing?

    I know that this project can be converted into an ammeter, I'd like to know if any of the above circuits (a & b) will measure current from 0 to 200 mA? Or better yet, 0 to 120 mA only. I have a circuit that supplies 0 to 120 mA to a 100 Ohm load and I want to know if this project can measure the current in the load to .1 decimal places perhaps.

    Is it a simple case of using a 10 Ohm resistor for R3 according to circuit b? Will that give me a 0 - 200 mA display if I hooked this up as an ammeter?

    Appreciate any replies.

    v5 out.

  4. Erm, thanks for that indulis, but I think I'm going to need a few other values and device numbers to go with that.

    Is that a precision constant current source circuit?

    Edit: I decided to simulate the circuit using SIMetrix, with a few components:


    ^ Thats my version of the circuit. It works OK for a constant 20 Ohms, however the current will change if I replace the 20 Ohm load. Also I simulated it under varying temperatures and there's a drift of ~+5 mA, whereas in the circuit I have (with the appropriate heatsink in place) will drift by only ~+0.01 mA.

    I do hope that its the way I have set it up thats not letting it work, I like the idea of using a MOSFET. I'd be grateful, indulis, if you could upload a schematic of your circuit working with those specifications I just made; perhaps you could adjust mine or change yours.

    One final thing: it'd be really awesome if you could get a current source to supply 0 to 120 mA to a 20 Ohm load, which is independent of temperature change. That would be just kick-ass. I'm thinking about perhaps hi-power op-amps that allow upto 150 or 200 mA.

  5. How would I go about creating a constant current source to supply a 20 Ohm load? I want the current to be variable from 0 to 120 mA.

    So I need a circuit which supplies a constant current to a 20 Ohm load, in the range of 0 to 120 mA.

    ^ That is a circuit that I have designed already, however I believe it is a little unstable and the transistor may need a heatsink to suffice the circuit.

    I was hoping that perhaps there is a way to achieve a circuit that is independent of temperature, so I won't need a heatsink or have to worry about possible thermal runaway.

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